1. Field of the Invention
The present invention relates to driver circuits in digital systems for adjusting input impedance and output impedance of respective receivers and transmitters relative to a prescribed impedance of a transmission line.
2. Background Art
Newer processor architecture designs require transfer of data between large integrated circuits at higher speeds. These higher speeds require that the integrated circuits have input/output impedances that precisely match the prescribed impedance of the PC board traces interconnecting the integrated circuits. In particular, impedance matching is needed to minimize signal reflections caused by a change in impedance at the interface between a signal trace on the PC board and an integrated circuit chip. For example, HyperTransport™ technology specifies a data rate of 1.6 Gbps between each wire pair based on a double data rate using an 800 MHz clock. Hence, precise impedance matching is needed between the signal traces (typically 50 Ohms) and the terminating devices to ensure data integrity in the high-speed digital systems, as well as reliability, faster speeds, and minimal power consumption.
One known technique to provide a matching impedance is to implement a driver circuit on the integrated circuit chip. The driver circuit is configured for providing a controlled, predetermined output impedance that reduces the effects of reflections on the transmission line. Two types of driver circuits can be implemented, namely a voltage mode driver circuit, and a current mode driver circuit. The voltage mode driver circuit typically have a relatively low output impedance, whereas current mode driver circuits have a relatively high output impedance.
Current mode driver circuits tend to be preferred because they do not require accurate accurate voltage mode references; however, a current mode driver circuit must draw from the supply a greater amount of current to implement a given logic voltage swing in comparison to a voltage mode driver circuit. Hence, power consumption is substantially higher in a current mode driver circuit than a voltage mode driver circuit.
Hence, a voltage mode driver circuit provides the advantage of reduced power consumption and a lower output impedance that tends to be closer to the 50 ohm impedance of the transmission line. The voltage mode driver circuit requires a high-precision voltage source that swings between two voltage values (“rails”) that represent the respective logic values. Hence, one disadvantage is that the voltage mode driver circuit requires accurate voltage references. The voltage mode driver circuit also is coupled in series with a precision resistor (i.e., a resistor having a tolerance of about 1 percent) having a resistance that matches the transmission line impedance.
However, a problem in implementing a voltage mode driver circuit is that substantial variations in process or manufacturing techniques, voltage supply, and operating temperatures may cause substantial variations in the silicon structure which that implements the digital driver circuitry that includes the voltage mode driver. Consequently, a voltage mode driver circuit on an integrated circuit, when interconnected among different components on a PC board, may encounter in the impedance variations by as much as 50 percent.
Hence, process variations during fabrication and temperature variations can change input/output impedances by a sufficient degree that static impedance matching techniques are insufficient.